1. Field of the Invention
Corrosion inhibitors for steel absorption cycle components using aqua ammonia working fluid.
2. Description of the Related Art
Various corrosion inhibition protocols are described in the prior art which protect mild steel in NH.sub.3 --H.sub.2 O absorption cycle service, and limit non-condensable gas formation.
McKelvy and Isaacs in Bureau of Standards Technologic Paper 180 (1920) disclosed the importance of eliminating carbonates, and also that adding 0.28 weight percent (w/o) of sodium or potassium dichromate (based on water content) stops any gas generation.
When it was later found that the chromate was depleted over time, greater amounts were added, until solubility limit problems were encountered. U.S. Pat. No. 2,580,983 disclosed provision of relatively insoluble SrCrO.sub.4 or PbCrO.sub.4 in tablet form, in conjunction with up to 2 w/o NaOH, to effect slow release of the chromate.
Alkali hydroxides have routinely been used for pH adjustment in conjunction with other inhibitors in LiBr absorption systems.
This has also been done in aqua ammonia absorption systems, for example, NTIS PB 95-226445 discloses 0.19 w/o NaOH plus 1 w/o sodium metasilicate as inhibitor. That amount of NaOH in the charge of distilled water produces a pH of 12.67 at 25.degree. C. The NaOH alone was found to be at least as effective as the silicate in inhibiting the 10 w/o NH.sub.3 solution.
Other investigators also advocate using alkali hydroxides as inhibiitor. U.S. Pat. No. 5,725,793 discloses alkali metal hydroxide concentrations in the range of 2 to 10 w/o (based on H.sub.2 O) as being effective for both corrosion inhibition and also for COP enhancement owing to reduced or eliminated need for rectification. This range of concentrations will yield pH in the range of 13.1 to 14.4 in the charged distilled water at 25.degree. C.
U.S. Pat. No. 5,811,026 discloses inhibiting aqueous ammonia with a strong base (alkali metal hydroxide) at a concentration between 0.015 Normal and 0.2 N at 25.degree. C., relative to the charge distilled water. This corresponds to a pH in the range of 12.2 to 13.3. The inhibitor is disclosed as effective for aqueous NH.sub.3 concentrations down to about 1 w/o, although the autoclave test results reported are all at 10 w/o NH.sub.3.
Modern advanced GAX cycles (U.S. Pat. Nos. 5,097,676, 5,024,063, and 5,572,885) must concentrate the water absorbent up to fairly high purity, with substantially less than 1% residual NH.sub.3, in order to achieve high performance. The prior art teachings of high caustic content are found to cause excessive corrosion and non-condensable gas generation at those conditions. Hence, an improved corrosion inhibitor is needed for advanced aqua ammonia absorption cycles. Thus, the objective of this invention is to provide a corrosion-inhibited aqua ammonia solution which not only inhibits corrosion and non-condensable gas generation in the high ammonia concentration environments of prior art generators (e.g., &gt;5 w/o NH.sub.3) but also in the generators of advanced absorption cycles wherein the NH.sub.3 concentration is less than 1%, and may be less than 0.1%.